1. Field of the Invention
This invention relates to a shielding apparatus used in case of constructing horizontal and vertical holes, tunnels or the like under the ground.
2. Description of the Prior Art
As one of shielding apparatuses for constructing tunnels or the like without excavating the ground by consolidating the ground as in case of using a pile driving machine, there is a disclosure of such an apparatus that comprises a cylindrical shield body, a crankshaft supported by the shield body rotatably around the axis of the shield body and having an eccentric portion, a rotor provided in front of the shield body and rotatably supported by the eccentric portion of the crankshaft, tile rotor having an external surface in an approximately conical or truncated conical form, and a driving mechanism for rotating the crankshaft around the axis of the shield body (Japanese Pat. Public Disclosure (KOKAI) No. 59-192193) corresponding to U.S. Pat. No. 4,624,605.
This shielding apparatus known per se is advanced by a pipe-propelling device disposed in the starting shaft or the like while the crankshaft is rotated by the driving mechanism. The rotor makes a revolving motion (revolution) around the rotational axis of the crankshaft in accordance with the rotation of the crankshaft, and also makes a rotary motion (rotation) about the eccentric portion of the crankshaft by rotating about the eccentric portion of the crankshaft while bringing the outer circumferential surface of the rotor into contact with the ground. Accordingly, the shielding apparatus known per se forms holes such as tunnels into the ground by being advanced while consolidating the ground by the outer surface of the rotor.
In this kind of shielding apparatus, a reaction force in the radial direction of the shield body or a hole to be formed is yielded on the rotor in accordance with the consolidation of the ground by the rotor. The reaction force acts on the shield body such that the shield body is pushed against the ground so as to yield a frictional force between the shield body and the ground when the shield body advances. The strength of such a frictional force gives a great influence on a thrust required for the advancement of the shield body.
However, since only one rotor is used, in the above-mentioned shielding apparatus known per se, the reaction force acts on the shield body as it is. As a result, a frictional force between the shield body and the ground is large, and therefore a large thrust is necessary for the advancement of the shield body. Furthermore, the above-mentioned reaction force acts on the shield body as a bending moment, and therefore the advancing direction of the apparatus is unstable.
As one of other apparatuses for constructing tunnels or the like without excavating the ground by consolidating the ground, there is a disclosure (U.S. Pat. No. 3,926,267) of such an apparatus that comprises a driving mechanism, a crankshaft rotated by the driving mechanism, and a plurality of rotors provided in front of the driving mechanism and rotatably supported by the crankshaft. The driving mechanism is provided with a casing and a plurality of projection portions protruding from the casing so as to touch on the inner surface of a hole formed by the revolving and rotary motions of the rotors.
In this apparatus known per se, the casing of the driving mechanism has an outer diameter smaller than the inner diameter of the hole to be formed. For this reason, in the process of the hole being forming, the hole around the casing is kept by the ground consolidated around the hole, and the rotational reaction force caused by the revolving and rotary motions of the rotors is transmitted to the ground through the projection portions.
In such an apparatus using the casing having the outer diameter smaller than the inner diameter of the hole to be formed in this manner, however, the casing does not contact the ground around the hole. Therefore, there is no problem brought by any frictional force caused by the reaction force in the radial direction of the hole.
However, in the apparatus using the casing having the outer diameter smaller than the inner diameter of the hole to be formed in this manner, since the casing has not action for preventing the ground about the hole from caving in, the ground about the hole cannot be prevented from caving in. Accordingly, the above-mentioned apparatus is not practical.